Liquid crystal polymer devices and materials

ABSTRACT

Methods for loading and alignment of liquid crystal polymers in electro-optic and electro-active devices comprise in-situ polymerisation between substrates of monomer material in the presence of a cationic initiator. This allows for the synthesis of liquid crystal polymers containing polyether backbones.

This invention relates to the alignment of liquid crystal polymers inliquid crystal devices and the provision of novel liquid crystalpolymers.

The unit that is the basic building block of a polymer is called amonomer.

The polymerisation process i.e. the formation of a polymer from itsconstituent monomers does not usually create polymers of uniformmolecular weight, rather what is created is a distribution of molecularweights. In order to describe a sample of polymer it is necessary tostate the average number of monomers in a polymer this is called thedegree of polymerisation (D.P). By how much the majority of polymermolecules differ from this average value (or to describe the spread ofmolecular weight) is called the polydispersity.

A number of different average molecular weights can be drawn from gelpermeation chromatography (GPC) for a given sample including: Mn--numberaverage molecular weight and Mw--weight average molecular weight. Thevalue used to calculate D.P. is usually Mn and polydispersity is usuallydefined as Mw/Mn.

Polymers can be made from different types of monomers, in which case thepolymer is called a co-polymer. If two types of monomer join in a randomfashion then the polymer is called a random co-polymer. If the twomonomers form short sequences of one type first which then combine toform the final polymer then a block copolymer results. If shortsequences of one of the monomers attach themselves as side chains tolong sequences consisting of the other type of monomer then the polymeris referred to as a graft copolymer.

In liquid crystal polymers the monomers can be attached together inessentially two ways. The liquid crystal part or mesogenic unit of thepolymer may be part of the polymer backbone resulting in a main chainpolymer, alternatively the mesogenic unit may be attached to the polymerbackbone as a pendant group i.e. extending away from the polymerbackbone; this results in a side-chain polymer. These different types ofpolymer liquid crystal are represented schematically below. Themesogenic units are depicted by the rectangles. ##STR1##

The side chain liquid crystal polymer can generally be thought of ascontaining a flexible polymer with rigid segments (the mesogenic unit)attached along its length by short flexible (or rigid) units as depictedin the schematic representation below. It is the anisotropic, rigidsection of the mesogzenic units that display orientational order in theliquid crystal phases. In order to affect the phases exhibited by theliquid crystal and the subsequent optical properties there are manyfeatures which can be altered, some of these features are particularlypertinent to side-chain liquid crystal polymers. One of these featuresis the flexible part that joins the mesogenic unit to the polymerbackbone which is generally referred to as a spacer, the length of thisspacer can be altered, its flexibility can also be altered. ##STR2##

A number of side-chain liquid crystal polymers are known, for examplesee GB 2146787 A.

Liquid crystal polyacrylates are a known class of liquid crystal polymer(LCP). LCPs are known and used in electro-optic applications, forexample in pyroelectric devices. non-linear optical devices and opticalstorage devices. For example see GB 2146787 and Makromol. Chem. (1985)186 2639-47.

Side-chain liquid crystal polyacrylates are described in PolymerCommunications (1988), 24, 364-365 e.g. of formula: ##STR3## where(CH₂)_(m) --X is the side-chain mesogenic unit and R is hydrogen oralkyl.

Side-chain liquid crystal polychloroacrylates are described in Makromol.Chem. Rapid Commun. (1984), 5,393-398 e.g. of formula: ##STR4## where Ris chlorine.

A method for the preparation of polyacrylate homo- or co-polymers havingthe following repeat unit is described in UK patent application GB9203730.8 ##STR5## R₁ and R₂ are independently alkyl or hydrogen, R₃ isalkyl, hydrogen or chlorine, m is O or an integer 1-20, W is a linkagegroup COO or OOC or O and X is a mesogenic group.

One of the main problems with polymer liquid crystals is that the, areextremely difficult to align in devices. Essentially there are twotechniques which have been used for aligning polymer liquid crystals. Itis possible to try to align the liquid crystal polymer in a similarmanner as a low molar mass liquid crystal, which is described in moredetail below. Alternatively, mechanical techniques can be used such asshearing. Typically mechanical shearing is performed over hot rollers,this technique is generally only suitable for flexible substrates. It ispossible to shear a sample between glass slides however the glass slidescannot be sealed in the conventional manner.

Materials and Assembling Process of LCDs by Morozumi in Liquid CrystalsApplications and uses, vol 1 Ed. Bahadur, World Scientific PublishingCo. Ptc. Ltd. 1990 pp 171-194 and references therein as the titlesuggests discusses methods for assembling liquid crystal devices.

The technique for aligning low molar mass liquid crystals is typicallyas follows. Transparent electrodes are fabricated on the surfaces of thesubstrates, the substrates typically being made of glass e.g. glassslides. In twisted nematic or super twisted nematic devices, forexample, an alignment process is necessary for both substrates. A thinalignment layer is deposited to align the liquid crystal molecules,typically either organic or inorganic aligning layers are used, forexample SiO deposited by evaporation is a typical inorganic alignmentlayer. One method to form the alignment layer involves rubbing thesurface by textures or cloths. Polyimides have also been employed forthe surface alignment of layers. Polymide is coated onto the substratesbearing electrodes by a spinner and then cured to form a layer ofapproximately 50 nm thickness. Then each layer surface is repeatedlyrubbed in substantially one direction with an appropriate material. Ifthe liquid crystal molecules are deposited on this layer they areautomatically aligned in the direction made by the rubbing. It is oftenpreferable if the molecules possess a small angle pre-tilt typically2-3°. Higher pre-tilts are sometimes required.

The two substrates are then fixed together for example by adhesive andare kept separate by spacing materials. This results in uniform andaccurate cell spacing. A typical adhesive is an epoxy resin. Thissealing material is usually then precured. The electrodes may then beprecisely aligned for example to form display pixels. The cell is thencured at, for example 100-150° C. At this point the empty liquid crystalcell is complete.

It is at this point that the cell is filled with liquid crystalmaterial. The opening size in the sealing area of the liquid crystalcell is rather small therefore the cell can be evacuated, for example ina vacuum chamber, and the liquid crystal forced into the cell via gaspressure. More than one hole in the sealing area may be used. The emptycell is put into a vacuum chamber and then the vacuum chamber is pumpeddown. After the cell has been evacuated the open region of the sealantis dipped into the liquid crystal material and the vacuum chamber isbrought back to normal pressure. Liquid crystal material is drawn intothe cell as a result of capillary action, external gases can be appliedto increase the pressure. When the filling process is complete the holeor holes in the sealant is/are capped and the cell is cured at atemperature above the liquid crystal material clearing point to make theliquid crystal molecular alignment stable and harden the cappingmaterial.

Polymer liquid crystal molecules tend to be more viscous than lowmolecular weight liquid crystal materials and are therefore moredifficult to align and more difficult to fill into devices. Only liquidcrystal polymers with low molecular weights can be flow filled into acell, and once a degree of polymerisation greater than around 30 or 40repeat units is reached, most liquid crystal polymers become so viscousthat flow filling cells is extremely difficult. Much slower cooling isneeded in order to try and align liquid crystal polymers and thisusually results in poor uniformity of alignment.

Poorly aligned liquid crystal molecules do not result in the fastswitching high contrast materials and devices that are generallyrequired.

The above techniques are suitable for many liquid crystal materialsincluding those devices which use liquid crystal materials which exhibitand utilise the smectic mesophase e.g. ferroelectrics. Suitablealignment techniques may also be found in GB 2210469 B.

Devices containing ferroelectric liquid crystal mixtures can exhibitfist switching times (faster than 100 μs), Clark and Lagerwall, Appl.Phys. Lett., 36, 89, 1980. They can be bistable which means that theycan be multiplexed at high levels using a line-at-a-time scan technique.Ferroelectric materials continue to receive a large amount ofinvestigative attention due to their application in high resolution flatpanel displays. An important feature of devices containing liquidcrystalline materials is that they should exhibit a fast response time.The response time is dependent on a number of factors, one of thesebeing the spontaneous polarisation, denoted Ps (measured in nC cm⁻²). Byadding a chiral dopant to the liquid crystalline mixture the value of Pscan be increased, thus decreasing the response time of the device.Ferroelectric smectic liquid crystal materials, which can be produced bymixing an achiral host and a chiral dopant, use the ferroelectricproperties of the tilted chiral smectic C, F, G, H, I, J, and K phases.The chiral smectic C phase is denoted S_(C) * with the asterisk denotingchirality. The S_(C) * phase is generally considered to be the mostuseful as it is the fastest switching. It is desirable that the materialshould exhibit a nematic (denoted N) and S_(A) phase at temperaturesabove the chiral smectic phase in order to assist surface alignment in adevice containing liquid crystalline material. Ferroelectric smecticliquid materials should ideally possess the following characteristics:low viscosity controllable Ps and an S_(C) * phase that persists over aa broad temperature range, which should include ambient temperature, andexhibits chemical and photochemical stability. Materials which possessthese characteristics offer the prospect of very fast switching liquidcrystal containing devices.

Ferroelectric LCDs by Dijon in Liquid Crystals Applications and Uses,vol 1 Ed. Bahadur, World Scientific Publishing Co. Pte. Ltd, 1990 pp350-360 and references therein discusses alignment processes for smecticphases for low molar mass materials. The filling of cells is believed tobe possible only in the isotropic or nematic phase due to the viscosityof smectic phases. Generally materials with the following phase sequencegive good alignment:

I--N*--S_(A) --S_(C) * or I--S_(A) --S_(C) *

whereas materials with the following phase sequences are more difficultto align:

I--N*--S_(C) *

Typically, therefore. in order to use a liquid crystal material in thesmectic phase it will involve heating the material to the nematic orisotropic phase and allowing it to cool slowly into an aligned smecticstate. Should this technique be applied to a polymer liquid crystalmaterial then the cooling time is usually very much longer in order toassist the alignment, though very often the alignment is poor.

This invention solves the above problems by the in-situ polymerisationof liquid crystal monomer material.

According to a first aspect of the invention

A method of making an electro-optic device comprises the steps of:

forming a cell comprising two cell walls spaced apart, the walls innersurfaces having formed thereon electrode structures,

providing a mixture comprising a monomer material and a cationicinitiator,

introducing the mixture between the cell walls, polymerising themixture.

There may also be present in the mixture a radical initiator.

Preferably at least one wall is surface treated to provide liquidcrystal alignment.

The monomer material may be aligned before polymerisation and/or thepolymer may be aligned after polymerisation. The monomer may be presentin any of the known liquid crystal phases including nematic, cholestericor smectic.

Preferably the polymerisation is carried out under UV light and/or inthe presence of additional heat.

The cell walls may be substantially rigid or at least one of them may besubstantially flexible; such a cell may be used to manufacture a thinlayer of liquid crystal polymer, eg a smectic liquid crystal polymer. Inthis method the electrodes are not necessary. An aligned layer of liquidcrystal polymer is produced as above. At least one of the substantiallyflexible cell walls is removed, eg by peeling away from the alignedpolymer layer. If required, electrodes can be formed on at least one ofthe layers; for example the electrodes may be made from Indium TinOxide, Aluminium or Gold. Electrodes may also be constructed fromconducting polymer or a combination of the above. The electrodes may ormay not be transparent. The layer or layer electrodes may be mountedonto one support or between supports. Such a method may be useful inproducing pyroelectric piezoelectric and other electro-active devices,for example sensors.

Patent Application GB 9420632.3 describes the use of chain transferreagents to control molecular weight of liquid crystal polymers. Theremay also be a chain transfer reagent present in the mixture of thecurrent invention.

According to a further aspect of the invention, material of formula I isprovided ##STR6## wherein X and X₁ are independently selected fromstraight or branched chain C₁ -₁₆ allyl, halogen and H; ##STR7## is anysuitable mesogenic group; Z=single covalent bond, oxygen, CO₂ or OCO;

n=1-20; Y=O, CO₂, OCO, CH₂, CHOH;

m=3-10,000;

the mesogenic group is defined from general structure II: ##STR8##wherein A, B and D are independently selected from: ##STR9## W₁ and W₂are independently selected from a single covalent bond. COO, OCO, CH₂CH₂, CH₂ O, OCH₂, O;

Q is selected from:

CN, halogen, R, OR, COOR, OOCR, CF₃ and lactate derivatives where R maybe chiral, straight or branched chain alkyl and may include from 1-16carbon atoms and including where one or more non-adjacent CH₂ groups maybe substituted by CH(CN), CH(CF₃), CH(Cl), CH(CH₃);

the substituents on the phenyl and cyclohexyl rings indicate that atleast one substituent may be present on the rings specified and up tofour substituents present on the phenyl rings and up to ten substituentspresent on the cyclohexyl rings.

Liquid crystal polymers described by the current invention may be any ofthe known types including homo or co polymers.

Y may be CHOH and the OH groups used as a point of attachment forcross-linking agents to form elastomers, examples of such cross-linkingagents include: ##STR10##

According to a further aspect of the invention a method of making anelectro-optic device comprising one or more materials of formula Icomprises the steps of:

forming a cell comprising two cell walls spaced apart, the walls innersurfaces having formed thereon electrode structures,

providing a mixture comprising a monomer material and a cationicinitiator,

introducing the mixture between the cell walls,

polymerising the mixture.

There may also be present in the mixture a radical photoinitiator.

Preferably at least one wall is surface treated to provide liquidcrystal alignment.

The monomer material may be aligned before polymerisation and/or thepolymer may be aligned after polymerisation. The monomer may be presentin any of the known liquid crystal phases including nematic, cholestericor smectic.

Preferably the polymerisation is carried out under UV light and/or inthe presence of additional heat.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described with reference to the followingdiagrams wherein

FIG. 1 is a synthetic scheme for the preparation of an epoxide monomer

FIG. 2 illustrates the polymerisation of an epoxide monomer

FIG. 3 illustrates a liquid crystal device

FIG. 4 illustrates a pyroelectric device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Scheme 1 refers to FIG. 1.

z/ benzyl chloride, K₂ CO₃, butanone, reflux 24 hr

a/ Mg, B(OMe)₃, THF, HCl(aq)

b/ (CH₃ CO)₂ O, H₂ O, NaOH, reflux 24 hr

c/ Pd(PPh₃)₄, K₃ PO₄.H₂ O, 1,4-dioxane, 80° C., N₂ (g), 24 hr

d/ KOH, IMS/H₂ O, reflux 24 hr

e/ CH₂ ═CH(CH₂)₉ Br, K₂ CO₃ /butanone, reflux 24 hr

f/ CH₃ (CH₂)₇ OH, PPh₃, DEAD, THF, 25° C., N₂ (g), 24 hr

g/ KOH, IMS/H₂ O, reflux, 24 hr

h/ H₂ (g), THF, Pd/C (5%), 25° C., 24 hr

i/ TFAA, DCM, 25° C., N₂ (g), 8 hr

j/ m-CPBA, DCM, 0° C., N₂ (g), 18 hr

wherein, DCM=dichloromethane, DEAD=diethyl azodicarboxylate,IMS=industrial methylatedspirit, m-CPBA=meta-chloroperbenzoic acid.TFAA=trifluoroacetic anhydride, THF=tetrahydrofuran.

4-bromophenol was purchased from Aldrich, 2-fluoro,4-bromo-phenol waspurchased from Wychem UK. All other starting materials were purchasedfrom Aldrich.

An example of the use of a material and device embodying the presentinvention will now be described with reference to FIG. 3.

The liquid crystal device consists of two transparent plates, 1 and 2,for example made from glass. These plates are coated on their internalface with transparent conducting electrodes 3 and 4. An alignment layeris introduced onto the internal faces of the cell so that a planarorientation of the molecules making up the liquid crystalline materialwill be approximately parallel to the glass plates 1 and 2. This is doneby coating the Lass plates 1,2 complete with conducting electrodes sothat the intersections between each column and row form an x, y matrixof addressable elements or pixels. Prior to the construction of the cellthe films 5,6 are rubbed with a roller covered in cloth (for examplemade from velvet) in a given direction, the rubbing directions beingarranged parallel (same or opposite direction) upon construction of thecell. A spacer 7 eg of polymethyl methacrylate separates the glassplates 1 and 2 to a suitable distance eg 2 microns. Liquid crystalmaterial 8 is introduced between glass plates 1,2 by filling the spacein between them. This may be done by flow filling the cell usingstandard techniques. The spacer 7 is sealed with an adhesive 9 in avacuum using an existing technique. Polarisers 10, 11 may be arranged infront of and behind the cell.

Alignment layers may be introduced onto one or more of the ccll walls byone or more of the standard surface treatment techniques such asrubbing, oblique evaporation or as described above by the use of polymeraligning layers.

The device may operate in a transmissive or reflective mode. In theformer, light passing through the device, eg from a tungsten bulb, isselectively transmitted or blocked to form the desired display. In thereflective mode a mirror, or diffuse reflector, (12) is placed behindthe second polariser 11 to reflect ambient light back through the celland two polarisers. By making the mirror partly reflecting the devicemay be operated both in a transmissive and reflective mode.

In an alternative embodiment a single polariser and dye material may becombined.

The liquid crystal material 8 when introduced into the cell consists ofliquid crystal monomers and a cationic initiator. It may also contain areagent which will limit the molecular weight of the polymer for examplea chain transfer reagent and it may also include a radicalphotoinitiator and/or thermal initiator.

The monomer material may be aligned before polymerisation using standardtechniques, for example by heating up to and cooling from the isotropicphase or from a liquid crystal phase such as a nematic or chiral nematicphase. It is also possible that the liquid crystal polymer may bealigned by one or more techniques including the use of surface forces,shear alignment or field alignment.

It is possible that following polymerisation there may still be someamount of monomer material remaining. This may be unreacted monomer orlow molar mass additives which do not bear polymerisable groups.

Polymerisation may be carried out by using any of the known techniques.For example the monomer material plus cationic initiator may alsocontain a photoinitiator and be exposed to UV light, heat may also beapplied to permit polymerisation within a given phase of the monomerand/or polymer.

Alternatively the polymerisation process may take place in the presenceof heat and a thermal initiator. However if this technique is used it ispreferable if it is carried out at a temperature which corresponds to aliquid crystal phase of the monomer material.

In order to polymerise the epoxide (see FIG. 2), several mixtures wereprepared. These mixtures consisted of the epoxide monomer illustratedbelow (referred to as RM 255), a radical photoinitiator Irgacure 651(available from Ciba Geigy) and a cationic photoinitiator. Theconcentration of cationic initiator was varied. The cationic initiatorused was Cyracure UVI-6974 (available from Union Carbide). ##STR11##

Transition temps/°C.

K 67.5 S 144.4 N 153.6 I

The mixtures were prepared in a brown glass sample tube and care wastaken to avoid excess sunlight. The total weight of the mixtures wasapproximately 100 mg. The radical photoinitiator Irgacure 651 wasweighed in to the sample tube and the calculated weight ofphotoinitiator was added. The mixture was then made up to the totalweight with the epoxide monomer. The mixture was then dissolved in a fewdrops of dichloromethane, mixed well and evaporated to dryness at roomtemperature under a stream of dry nitrogen. The mixtures were then readyto be polymerised.

Polymerisations were carried out on thin samples e.g. the monomermixture between glass microscope slides. The monomer mixture in thesample tube was melted gently into the isotropic phase with a hot airblower, a small sample was removed on a glass rod and placed on apreheated glass slide on a hotplate at a temperature a little above theclearing point of the monomer (˜160° C.). A second glass slide was thenplaced on top and gently pressed to give a thin film of monomer mixture.The samples were then removed from the hotplate and were ready forpolymerisation.

The prepared samples were placed on a hot plate at the desiredtemperature, covered with thick card and allowed to come to equilibrium(˜5 mins). Meanwhile the desired UV lamp above the hotplate was turnedon and also allowed to warm to a constant state. The card was removedfor the exposure time and then replaced. The lamp was then turned offand the polymerised samples removed from the hotplate.

The polymers were all examined by GPC to determine the extent ofpolymerisation. The method for this was as follows:

The glass slides containing the polymers were warmed to allow the twoslides to be separated. The slides were then washed with THF to removeall polymer and monomer. The THF solutions were then examined by GPC.The relative areas of the polymer and monomer peaks were used todetermine the extent of polymerisation.

The following mixture was investigated:

RM255 98.5%

Irgacure 651 0.5%

Cyracure UVI 6974 1.0%.

The following results were obtained at varying exposure times with a DrHonle lamp (˜50m W/cm²) at a temperature of 110° C.

    ______________________________________                                               Time/min                                                                             % polymer                                                       ______________________________________                                                5     74.4                                                                   10     63.7                                                                   20     74.4                                                                   30     68.4                                                                   80     84.5                                                            ______________________________________                                    

Different curing temperatures were investigated for the followingsystem:

RM255 97.3%

Irgacure 651 0.8%

Cyracure UVI-6974 1.8%

    ______________________________________                                        Temp/° C.                                                                            time/mins                                                                              % polymer                                              ______________________________________                                         89           5        63                                                      99           5        82                                                     111           5        80                                                     125           5        77                                                     136           5        75                                                     143           5        78                                                     154           5        60                                                     166           5        <59                                                    180           5        ˜50                                              ______________________________________                                    

Different exposure times were investigated for the following system:

RM255 98.5%

Irgacure 651 0.5%

Cyracure UVI-6974 1.0%

    ______________________________________                                        Temp/° C.                                                                            Time/mins                                                                              % polymer                                              ______________________________________                                        110            5       74.4                                                   110           10       63.7                                                   110           20       74.4                                                   110           30       68.4                                                   110           80       84.5                                                   ______________________________________                                    

Different exposure times were investigated for the following system:

RM255 97.3%

Irgacure 651 0.8%

Cyracure UVI-6974 1.8%

    ______________________________________                                        temp/° C.                                                                            time/mins                                                                              % polymer                                              ______________________________________                                        110             0.5    <50                                                    110            1       62.5                                                   110            2       65.2                                                   110            5       78.0                                                   110           10       80.4                                                   110           15       74.8                                                   110           20       86.0                                                   110           30       82.7                                                   110           60       83.0                                                   110           120      81.8                                                   ______________________________________                                    

Different exposure times were investigated for the following system:

RM255 90.8%

Irgacure 651 2.8%

Cyracure UVI-6974 6.4%

    ______________________________________                                        temp/° C.                                                                            time/mins                                                                              % polymer                                              ______________________________________                                        110             0.5    75.8                                                   110            1       77.2                                                   110            2       75.6                                                   110            5       80.1                                                   110           10       79.3                                                   110           15       81.0                                                   110           20       78.6                                                   110           30       80.7                                                   110           60       79.9                                                   ______________________________________                                    

Materials have been proposed for laser addressed applications in whichlaser beams are used to scan across the surface of the material or leavea written impression thereon. For various reasons many of thesematerials have consisted of organic materials which are at leastpartially transparent in the visible region. The technique relies uponlocalised absorption of laser energy which causes localised heating andin turn alters the optical properties of the otherwise transparentmaterial in the region of contact with the laser beam. Thus as the beamtraverses the material, a written impression of its path is left behind.One of the most important of these applications is in laser addressedoptical storage devices, and in laser addressed protection displays inwhich light is directed through a cell containing the material and isprojected onto a screen. Such devices have been described by Khan Appl.Phys. Lett. vol. 22, p111, 1973: and by Harold and Steele in Proceedingsof Euro display 84, pages 29-31, September 1984. Paris. France, in whichthe material in the device was a smectic liquid crystal material.Devices which use a liquid crystal material as the optical storagemedium are an important class of such devices. The use of semiconductorlasers. especially Ga_(x) Al_(1-x). As lasers where x is from 0 to 1,and is preferably 1, has proven popular in the above applicationsbecause they can provide laser energy at a range of wavelengths in thenear infra-red which cannot be seen and thus cannot interfere with thevisual display, and yet can provide a useful source of well-defamed,intense heat energy. Gallium arsenide lasers provide laser light atwavelengths of about 850 nm, and are useful for the above applications.With increasing Al content (x<1), the laser wavelength may be reduceddown to about 750 nm. The storage density can be increased by using alaser of shorter wavelength.

The compounds of the present invention may be suitable as opticalstorage media and may be combined with dyes for use in laser addressedsystems. for example in optical recording media.

The smectic and/or nematic properties of the materials described by thecurrent invention may be exploited. For example the materials of thepresent invention nay be used in ferroelectric mixtures and devices.

The compounds of the present invention may also be used in pyroelectricdevices for example detectors, steering arrays and vidicon cameras.

FIG. 4 illustrates a simple pyroelectric detector in which the materialsof the present invention may be incorporated.

A pyroelectric detector consists of electrode plates 1,2 at least one ofwhich may be pixellated. In operation the detector is exposed toradiation R, for example infrared radiation, which is absorbed by theelectrode 1. This results in a rise in temperature which is transmittedto a layer of pyroelectric material 3 by conduction, The change intemperature results in a thermal expansion and a charge is generated.This change in charge is usually small when compared with the chargeoutput due to the change in the spontaneous polarisation, Ps with achange in temperature. this constitutes the primary pyroelectric effect.A change in charge results in a change in potential difference betweenthe electrodes. The charge on each pixel may be read out and theresulting signal is used to modulate scanning circuits in, for example,a video monitor and for a visual image of the infra red scans.

We claim:
 1. A liquid crystal polymer of the formula: ##STR12## whereinX and X₁ are independently selected from straight or branched chainC₁₋₁₆ alkyl, halogen and H; ##STR13## is any suitable mesogenic group;Z=single covalent bond, oxygen, CO₂ or OCO;Y is CHOH; n=1-20 andm=3-10,000.
 2. A liquid crystal polymer according to claim 1, whereinthe mesogenic group is defined from general structure II: ##STR14##wherein A, B and D are independently selected from: ##STR15## W₁ and W₂are independently selected from a single covalent bond, COO, OCO, CH₂CH₂, CH₂ O, OCH₂, O;Q is selected from:CN, halogen, R, OR, COOR, OOCR,CF₃ and lactate derivatives, where R may be chiral, straight or branchedchain alkyl and may include from 1-16 carbon atoms and including whereone or more non-adjacent CH₂ groups may be substituted by CH(CN),CH(CF₃), CH(Cl), CH(CH₃); the substituents on the phenyl and cyclohexylrings indicate that at least one substituent may be present on the ringsspecified and up to four substituents present on the phenyl rings and upto ten substituents present on the cyclohexyl rings.
 3. A liquid crystalpolymer according to claim 1 wherein the OH groups are used as a pointof attachment for cross-linking agents to form elastomers.
 4. A liquidcrystal material having optically active properties and containing atleast one liquid crystal polymer or copolymer as claimed in claim
 1. 5.A liquid crystal material according to claim 4 wherein the material is aferroelectric liquid crystal material.
 6. A liquid crystalelectro-optical display device characterised in that it includes thematerial as claimed in either of the claims
 4. 7. A device comprisingtwo spaced cell walls each bearing electrode structures and treated onat least one facing surface with an alignment layer, a layer of a liquidcrystal material enclosed between the cell walls, characterised in thatit incorporates the liquid crystal material as claimed in claim
 4. 8. Apyroelectric device comprising two spaced electrodes and a layer of aliquid crystal material enclosed between the electrodes, characterisedin that it incorporates the liquid crystal material as claimed in claim4.
 9. An optical recording medium comprising a recording a layer whichcomprises one or more compounds of claim 1 and a dye material.